1. Field of the Invention:
The present invention relates to an outboard marine propulsion unit or an outboard engine unit, and more particularly to an outboard engine unit having an engine with a vertically extending crankshaft.
2. Description of the Relevant Art:
Outboard engine units detachably mounted on the stern of small ships such as boats should preferably be compact so that they can be attached and detached with ease. Particularly, the transverse dimension of outboard engine units should be as small as possible since some applications require a plurality of outboard engines to be mounted side by side on the stern of a boat for increased propulsion forces.
An outboard engine unit has a casing assembly which includes an upper engine cover which houses an engine therein. Therefore, the upper engine cover has the greatest transverse dimension among other parts of the outboard engine unit. It is known that there are many types of accessories disposed around the engine, the accessories having different shapes and sizes. Particularly, intake/fuel supply devices which are disposed on one side of the engine are relatively large in size as compared with the other engine accessories. Therefore, if the engine is positioned centrally in the engine cover, the casing assembly must be large enough to provide a space for the installation of the engine therein. However, since the large casing assembly also creates a dead space on the other side of the engine, the overall outboard engine unit is necessarily large in size. The above problem manifests itself if large-capacity intake/fuel supply devices are associated with the engine for higher engine output power.
Japanese Laid-Open Patent Publication No. 60-38293 and Japanese Laid-Open Utility Model Publication No. 60-95293, for example, each disclose an outboard engine unit including an engine whose crankshaft extends vertically. The engine has cylinders whose axes are inclined, when the outboard engine unit is viewed in plan, with respect to the longitudinal central axis of the outboard end unit which is aligned with the direction in which propulsion forces are generated by the outboard engine unit. With this arrangement, a space defined on one side of the engine within the casing assembly is larger, and a space defined on the other side of the engine within the casing assembly is smaller. Intake/fuel supply devices are disposed in the larger space. Consequently, the space available for the installation of accessories in the casing assembly is effectively utilized, and the outboard engine unit is rendered compact.
As described above, in cases where the cylinder axes are inclined with respect to the longitudinal central axis of the casing assembly, the spaces on the opposite sides of the distal ends of the cylinders, which are remote from the crankshaft, can appropriately be adjusted for efficient space utilization. However, the space around the proximal ends of the cylinders, near the crankshaft, cannot effectively be utilized because the proximal end of the cylinders are positioned transversely centrally in the casing assembly.
The inclined cylinder axes are problematic especially if the engine is large in size. More specifically, if the cylinder axes are inclined with respect to the direction in which the propulsion forces are produced, then vibrations caused by the engine due to reciprocating movement of the pistons are appreciable with large engine sizes. Therefore, some considerations or measures are required to cancel out such vibrations which would be transmitted from the engine to the hull of the boat.